Wiped it clean.
Since we'd just had an argument, and taking away his computer for a day was part of his discipline for his disrespect, I feared he would think I had done it on purpose. I am not a revengeful mother.
So when I told him this morning, by then I had already begun thinking about the solution. He was hardly upset and said most of his stuff was backed up on the external hard drive anyway and we could just recopy it back onto his hard drive.
Ah, technology. My crisis creator and saving grace. If only my brain worked as well.